Accrington brick detached property cavity tray issue

My house is a 1930’s built Accrington brick detached property. I have recently had a small extension built to the side. The actual plan specifications stated the fitting of a cavity tray between the new and existing buildings but apparently this was not done. The extension which has a solid concrete floor, whereas my original house is suspended floorboards.
There was an initial problem with rising damp that my builders thought was due to a defect in the DPC of the original house in one particular area which they fixed by removing those bricks and relaying.
My concern is I have just removed the laminate flooring in my front room to which the extension was built and now opens into and find that the floorboards especially around the chimney breast are really damp with obvious signs of water staining. Basically the wood seems sound with little sign of any soft rotting but there is a musty smell and certainly mould growths were apparent under the old decor.